Transducers generally convert electrical signals to mechanical signals or vibrations, and/or mechanical signals or vibrations to electrical signals. Acoustic transducers, in particular, convert electrical signals to acoustic waves and acoustic waves to electrical signal using inverse and direct piezo-electric effects. Acoustic transducers generally include acoustic resonators, such as surface acoustic wave (SAW) resonators or bulk acoustic wave (BAW) resonators, and may be used in a wide variety of electronic applications, such as cellular telephones, personal digital assistants (PDAs), electronic gaming devices, laptop computers and other portable communications devices. Generally, BAW resonators in which a resonator stack is formed over a cavity are referred to thin film bulk acoustic resonators (FBARs), and BAW resonators in which the resonator stack is formed of over an acoustic reflector are referred to as solidly mounted resonators (SMRs). For example, FBARs and SMRs may be used for electrical filters and voltage transformers. Generally, an acoustic resonator has a layer of piezoelectric material between two conductive plates (electrodes), which may be formed on a thin membrane. BAW resonator devices, in particular, generate acoustic waves that can propagate in all possible lateral directions when stimulated by an applied time-varying electric field of finite lateral extension, as well as higher order harmonic mixing products. The laterally propagating modes and the higher order harmonic mixing products may have a deleterious impact on functionality.
In BAW resonators, mitigation of acoustic losses at the boundaries and the resultant mode suppression and confinement in the active region of the BAW resonators (the region of overlap of the top electrode, the piezoelectric layer, and the bottom electrode) has been effected through various methods. For example, frames are provided along one or more sides of the BAW resonators. The frames suppress the amplitude of electrically excited piston mode and simultaneously create an acoustic impedance mismatch that suppresses upon reflection propagating modes in the frame region, thus improving the confinement of the piston mode within the active region of the BAW resonator. However, better acoustic energy confinement, as well as further improvements in quality factor Q due to the better acoustic energy confinement, is needed for increased efficiency of BAW resonators, particularly with respect to SMRs.